netdev - Re: [PATCH 3/3] vhost-net: use lock_sock_fast() in peek_head

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Date:	Sun, 13 Mar 2011 16:52:50 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>, virtualization@...ts.osdl.org,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] vhost-net: use lock_sock_fast() in peek_head_len()

Le dimanche 13 mars 2011 à 17:06 +0200, Michael S. Tsirkin a écrit :
> On Mon, Jan 17, 2011 at 04:11:17PM +0800, Jason Wang wrote:
> > We can use lock_sock_fast() instead of lock_sock() in order to get
> > speedup in peek_head_len().
> > 
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > ---
> >  drivers/vhost/net.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index c32a2e4..50b622a 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -211,12 +211,12 @@ static int peek_head_len(struct sock *sk)
> >  {
> >  	struct sk_buff *head;
> >  	int len = 0;
> > +	bool slow = lock_sock_fast(sk);
> >  
> > -	lock_sock(sk);
> >  	head = skb_peek(&sk->sk_receive_queue);
> >  	if (head)
> >  		len = head->len;
> > -	release_sock(sk);
> > +	unlock_sock_fast(sk, slow);
> >  	return len;
> >  }
> >  
> 
> Wanted to apply this, but looking at the code I think the lock_sock here
> is wrong. What we really need is to handle the case where the skb is
> pulled from the receive queue after skb_peek.  However this is not the
> right lock to use for that, sk_receive_queue.lock is.
> So I expect the following is the right way to handle this.
> Comments?
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 0329c41..5720301 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -213,12 +213,13 @@ static int peek_head_len(struct sock *sk)
>  {
>  	struct sk_buff *head;
>  	int len = 0;
> +	unsigned long flags;
>  
> -	lock_sock(sk);
> +	spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
>  	head = skb_peek(&sk->sk_receive_queue);
> -	if (head)
> +	if (likely(head))
>  		len = head->len;
> -	release_sock(sk);
> +	spin_unlock_irqrestore(&sk->sk_receive_queue.lock, flags);
>  	return len;
>  }
>  

You may be right, only way to be sure is to check the other side.

If it uses skb_queue_tail(), then yes, your patch is fine.

If other side did not lock socket, then your patch is a bug fix.



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